Lesmahagow High School
Higher Chemistry
Oxidation of Food
Lesmahagow High School
CfE Higher Chemistry
Nature’s Chemistry
Oxidation of Food
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Lesmahagow High School
No.
Higher Chemistry
Oxidation of Food
Learning Outcome
1
Branched-chain alcohols with no more
than 8 carbon atoms in the longest
chain can be named from structural
formulae.
Understanding?
  
Given the names of branched-chain
alcohols, structural formulae can be
drawn and molecular formulae written.
  
3
Alcohols can be classified as primary,
secondary or tertiary.
  
4
Primary alcohols are oxidised, first to
aldehydes and then to carboxylic acids.
  
5
Secondary alcohols are oxidised to
ketones.
  
When applied to carbon compounds,
oxidation results in an increase in the
oxygen to hydrogen ratio.
  
Hot copper (II) oxide or acidified
dichromate (VI) solutions can be used
to oxidise primary and secondary
alcohols.
  
Tertiary alcohols cannot be oxidised.
  
2
6
7
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Higher Chemistry
Oxidation of Food
Branched-chain carboxylic acids, with
no more than 8 carbon atoms in the
longest chain, can be named from
structural formulae.
Given the names of branched-chain
carboxylic acids, structural formulae
can be drawn and molecular formulae
can be written.
  
Oxygen reacts with edible oils giving
the food a rancid flavour.
  
Antioxidants are molecules which will
prevent these oxidation reactions
taking place.
  
Ion-electron equations can be written
for the oxidation of many antioxidants.
  
  
Lesmahagow High School
Higher Chemistry
Oxidation of Food
Alcohols
Alcohols make up a group of organic compounds which contain
the -OH group, called the hydroxyl group. The -OH (or -O-H)
group is known as a functional group as this gives the molecules
its specific properties. The hydroxyl group is attached to
carbon together with atoms of hydrogen.
The presence of the hydroxyl group in these compounds is
indicated by the '-ol' ending of the name of the alcohol.
Alcohol
Molecular
Formula
Shortened Structure
Methanol
CH3OH
CH3OH
Ethanol
C2H5OH
CH3CH2OH
Propanol
C3H7OH
CH3CH2CH2OH
Butanol
C4H9OH
CH3CH2CH2CH2OH or
CH3(CH2)3OH
Pentanol
C5H11OH
CH3(CH2)4OH
Hexanol
C6H13OH
CH3(CH2)5OH
Heptanol
C7H15OH
CH3(CH2)6OH
Octanol
C8H17OH
CH3(CH2)7OH
As the alcohols are based on a family of hydrocarbons called
alkanes, these alcohols can also be called alkanols. The alkanols
are a homologous series with the general formula CnH2n+1OH
Isomers exist for alkanols, and the presence of the -OH or
hydroxyl group increases the possibility of isomers because
the position of the hydroxyl group can vary. You will be
expected to name and draw straight chain and branched
chain isomers up to C8.
When naming alkanols, the longest chain containing the
hydroxyl group is named first and numbered so that the
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
hydroxyl group in attached to the lowest numbered carbon
atom.
The shortened structural formula for 2,2-dimethylpropan-1-ol
is CH3C(CH3)2CH2OH
How to name an alcohol from the structural formula:
1. Look for the longest continuous chain of carbon atoms on
which the -OH group it attached and name this as the
parent compound.
2. The parent name is placed at the end of the name of the
compound.
3. Look for groups which form branches on the chain and
identify them.
4. Different groups are written in alphabetical order before
the parent name e.g. 'ethyl' before 'methyl'.
5. If there are 2 identical groups the prefix 'di' is placed
before the name of the branch e.g. 'dimethyl', 'diethyl'
etc
6. The prefixes 'tri' and 'tetra' are used if there are 3 or
4 repetitions respectively of the same group on the
parent chain.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
7. To specify the position of each group, the parent chain is
numbered from the end which results in the lowest
possible numbers in the formula.
Drawing structural formula using the alcohol name:
If you were asked to draw the full structural formula of 3methylpentan-2-ol, you would recognise it was an alcohol
(alkanol) from the '-ol' end, that it has 5 carbon atoms in its
chain (from the 'pentan' part of the name) and that it has a
methyl branch on the 3rd carbon (from the 3-methy start of
the name).
Draw a line of 5 carbon atoms with bonds between each C atom.
Put the other bonds to each carbon atom (4 in total on each C
atom). Count from the end of the chain to find the 2nd carbon
atom. To one of the bonds on this atom, attach an -OH group
(or an -O-H group to show all of the bonds needed in a full
structure. Count from the same end of the chain to find the
3rd carbon atom. To one of the bonds on the atom, attach a CH3 group (the methyl branch). Finally put H atoms on the ends
of all of the other bonds not connected to carbon atoms.
Classification of Alcohols
Alcohols can be classified in 3 groups.
1. Primary Alcohols (1°) – the carbon atom to which the
hydroxyl group is attached to is bonded to no more than
one other carbon. The other bonds are to hydrogen
atoms.
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Higher Chemistry
Oxidation of Food
2. Secondary Alcohols (2°) – the carbon atom to which the
hydroxyl group is attached to is bonded to two other
carbon atoms. Only one bond is to a hydrogen atom.
3. Tertiary Alcohols (3°) – the carbon atom to which the
hydroxyl group is attached to is bonded to three other
carbon atoms. There are no bonds to hydrogen atoms.
Oxidation of Alcohols
The oxidation of an organic compound causes the oxygen to
hydrogen ratio within the molecule to increase. This can be
achieved by either adding oxygen to or removing hydrogen
from the molecule.
In the reduction of an organic compound the oxygen to
hydrogen ratio is decreased by removing oxygen or adding
hydrogen.
Complete oxidation of an organic compound is also known as
combustion. When an alcohol is combusted carbon dioxide and
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
water are produced. Some organic compounds like alcohols can
be partially oxidised to produce different organic compounds.
Primary Alcohols can be oxidised in two stages. The first
stage changes the alcohol to an aldehyde.
Primary Alcohol
Aldehyde + water
Oxidising agent
In the second stage of oxidation, the aldehyde is further
oxidised to produce a carboxylic acid.
Aldehyde
Carboxylic acid
Oxidising agent
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Higher Chemistry
Oxidation of Food
Secondary Alcohols can be oxidised in one stage to produce
ketones.
Secondary Alcohol
Ketone + water
Oxidising Agent
Tertiary Alcohols cannot be oxidised readily.
In order to oxidise a primary or secondary alcohol an
appropriate oxidising agent should be used. Commonly used
oxidising agents are listed below.
 Acidified potassium dichromate solution
 Acidified potassium permanganate solution
 Heated solid copper (II) oxide
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
Carboxylic acids
Carboxylic acids contain the carboxyl functional group, -COOH
and can be recognised from their name by the '-oic' ending in
the name.
If this group is also attached to an alkane chain, the carboxylic
acid is called an alkanoic acid.
Alkanoic acids, CnH2n+1COOH
One exception to this general formula is methanoic acid,
HCOOH.
Alkanoic acid
Molecular
Formula
Shortened Structure
Methanoic
acid
HCOOH
HCOOH
Ethanoic acid
CH3COOH
CH3COOH
Propanoic
acid
C2H5COOH
CH3CH2COOH
Butanoic acid
C3H7COOH
CH3CH2CH2COOH or
CH3(CH2)2COOH
Pentanoic
acid
C4H9COOH
CH3(CH2)3COOH
Hexanoic
acid
C5H11COOH
CH3(CH2)4COOH
Heptanoic
acid
C6H13COOH
CH3(CH2)5COOH
Octanoic acid
C7H15COOH
CH3(CH2)6COOH
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
You will be expected to name straight-chain and branchedchain carboxylic acids, with up to eight carbon atoms in
their longest chain given their structural formulae.
You will also be expected to draw structural and write
molecular formulae when you are given the names of
straight-chain or branched-chain carboxylic acids
Some examples of full structures and names are shown below
How to name a carboxylic from the structural formula:
1. Look for the longest continuous chain of carbon atoms on
which the -COOH group it attached and name this as the
parent compound. The carbon atom in the carboxyl group
is included in the parent name of the compound e.g. a twocarbon chain attached to a carboxylic acid group would
give the parent compound the name 'propanoic acid'
2. The parent name is placed at the end of the name of the
compound.
3. Look for groups which form branches on the chain and
identify them.
4. Different groups are written in alphabetical order before
the parent name e.g. 'ethyl' before 'methyl'.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
5. If there are 2 identical groups the prefix 'di' is placed
before the name of the branch e.g. 'dimethyl', 'diethyl'
etc
6. The prefixes 'tri' and 'tetra' are used if there are 3 or
4 repetitions respectively of the same group on the
parent chain.
7. To specify the position of each group, the parent chain is
numbered such that the carbon atom in the carboxyl
group is C1.
Drawing structural formula using the carboxylic acid name:
If you were asked to draw the full structural formula of 2methylbutanoic acid, you would recognise it was an carboxylic
acid (alkanoic acid) from the '-oic' end, that it has 4carbon
atoms in its chain (from the 'butan' part of the name) and that
it has a methyl branch on the 2rd carbon (from the 2-methy
start of the name).
Draw a line of 4 carbon atoms with bonds between each C atom.
Put the other bonds to each carbon atom (4 in total on each C
atom). To one of the bonds on an end carbon atom, attach an OOH group (or
group to show all of the bonds needed in
a full structure. Count from the same end of the chain to find
the 2rd carbon atom. To one of the bonds on the atom, attach a
-CH3 group (the methyl branch). Finally put H atoms on the
ends of all of the other bonds not connected to carbon atoms.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
Oxidation of Foods
Oxidation of food can occur when food is exposed to oxygen in
the air. Foods which contain fats and oils are at a particularly
high risk of oxidation.
Oxidation reactions involving oxygen molecules from the
air damage the structure of the fat or oil causing
degradation of long fatty-acid chains and formation of
short-chain oxidation products. The oxidation of
unsaturated oils and fats primarily takes place via a
free-radical-mediated process and can lead to rancidity
(rancidus [Latin] = stinking), which negatively affects both
odour and taste, and has an impact on safety for human
consumption.
Fat
Fat
Oxygen
Reducing the rate of oxidation of foodstuffs
The rate of oxidation of foodstuffs can be slowed down by
reducing the temperature by refrigeration. Packaging
under a vacuum or under an inert gas such as nitrogen can
also reduce the oxidation rate by reducing the
concentration of oxygen (air). Crisp manufacturers fry
potatoes under a blanket of steam to reduce the oxygen
concentration, thus extending the lifetime of both the
frying oil and of the crisps.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
Although all of these methods of reducing oxidation are
fairly effective, it is still usual to add antioxidant(s) to
foodstuffs to prolong their shelf-life.
Antioxidants are molecules that reduce the rate of
oxidation reactions involving the transfer of electron(s)
to an oxidising agent. Antioxidants are often added to
foodstuffs to minimise oxidative damage.
An effective classroom demonstration to illustrate the
benefits of antioxidants in foodstuffs involves cutting an
apple in half and leaving one side exposed to the oxygen in
the air. The other side is coated in lemon juice, which
contains a high concentration of the antioxidant vitamin C.
After a few hours, the untreated half becomes brown due
to free-radical-induced oxidative damage, whereas the
treated half remains undamaged due to the antioxidant
properties of the vitamin C.
Oxidation
occurs when
the apple is
left exposed
to air
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The apple is protected
when dipped in orange
juice containing the
antioxidant vitamin C
Lesmahagow High School
Higher Chemistry
Oxidation of Food
How Antioxidants Work
When edible fats and oils are oxidised free radicals are
produced. A free radical is a highly reactive species which
contains an unpaired electron. Free radicals can damage
food by removing electrons. Antioxidants work by
“mopping-up” free radicals and so protecting the food.
This happens when antioxidant molecules donate an
electron to the free radical. This forms a stable pair of
electrons, stabilizing the free radical. During this process
the antioxidant molecule become oxidised as it loses an
electron. Ion-electron equations can be used to show
how antioxidant molecules are oxidised.
C6H8O6
Vitamin C
(ascorbic acid)
C6 H 6 O 6 + 2H + +
2e DHA
(dehydroascorbic acid)
Sources of Antioxidants
Antioxidants can be natural or synthetic. Natural
antioxidants tend to be short-lived and therefore
synthetic antioxidants are used when a longer shelf-life is
preferred. In reality, several antioxidants are often added
in combination to foodstuffs to give the most effective
action.
Vitamin C and its salts are added to soft drinks, jams,
condensed milk and sausage to prevent oxidation. The
tocopherols are members of the vitamin E family and are
found mainly in nuts, sunflower seeds, soya and maize
shoots. They are mostly used for preserving vegetable oils,
margarine and cocoa products.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
Antioxidant
Natural/synthetic
E
number
Types of
food
Vitamin C
Natural
E300
Fruits, jams,
vegetables
Natural
E306
Oils, meat
pies, soya
beans
Synthetic
E320
Margarine,
cheese,
crisps
(ascorbic acid)
Vitamin E
(tocopherols)
Butylated
hydroxyanisole
(BHA)
.
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Lesmahagow High School
Higher Chemistry
Oxidation of Food
Oxidation of Food - Glossary
Word
Meaning
Alcohols
Primary (1°)
Alcohols
Organic compounds containing the hydroxyl (-OH)
group.
Alcohols in which the hydroxyl is bonded to carbon
which is bonded to no more than one other carbon.
Secondary (2°)
Alcohols
Alcohols in which the hydroxyl is bonded to a carbon
which is bonded to two other carbons.
Tertiary (3°)
Alcohols
Alcohols in which the hydroxyl is bonded to a carbon
which is bonded to three other carbons.
Oxidation
A reaction in which an organic compounds has the
oxygen to hydrogen ratio increased. Oxygen can be
added or hydrogen can be removed.
A reaction in which an organic compound has the
oxygen to hydrogen ratio reduced. Oxygen can be
removed or hydrogen can be added.
Organic compounds containing the carboxyl
(-COOH) group.
Deterioration of flavour and odour.
Reduction
Carboxylic Acids
Rancidity
Free Radical
Antioxidants
Oxidising Agent
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Extremely reactive molecules with an unpaired
electron.
Molecules which donate an electron to free-radicals,
preventing the deterioration of food.
A substance which causes oxidation of another
substance and so undergoes reduction.